Propulsion systems for traction vehicles such as locomotives commonly use a diesel engine prime mover to drive electric generating means for supplying energy to a plurality of direct current (DC) traction motors. The generating means typically comprises a 3-phase dual output alternator where each output comprises three phase windings interconnected in a 3-phase star configuration. The alternator voltages are rectified and applied to relatively positive and negative DC buses between which the respective pairs of motors which are generally connected in parallel.
Referring now to FIG. 1, which depicts a typical series-parallel alternator-rectifier system 10 comprising a dual winding AC power supply (e.g. alternator) 200 interconnected with a rectifier assembly 14 including series-parallel switches 46 and 48. The three different phases of the first set of windings 210 are respectively identified as 222, 224, and 226, which may typically represent what is commonly termed as phases A, B, and C respectively. Likewise, the three different phases of the second set of windings 220 are identified as 212, 214, and 216, which again, may typically represent what is commonly termed as phases A prime, B prime, and C prime respectively for the second set of windings 220.
The rectifier assembly 14 is formed by an array of rectifiers or rectifiers, which are interconnected and arranged between the dual winding AC power supply 200 and the positive DC bus 100p and negative DC bus 100n. In FIG. 1, the rectifier assembly 14 has two series-parallel switches 46 and 48 and three primary legs connected in parallel circuit relationship between the DC buses. Each leg of the three primary legs comprises a four rectifiers (e.g. diodes) connected in series with one another and oriented to conduct current in a direction from negative DC bus 100n to positive DC bus 100p. A first primary leg 120 comprises a first rectifier 22, a second rectifier 24, a third rectifier 52, and a fourth rectifier 54 connected in series with one another and oriented to conduct current in a direction from negative DC bus 100n to positive DC bus 100p. The second primary leg 130 comprises a first rectifier 30, a second rectifier 32, a third rectifier 60, and a fourth rectifier 62 similarly connected and oriented. Finally, the third primary leg 140 also comprises a first rectifier 38, a second rectifier 40, a third rectifier 68, and a fourth rectifier 70 in the same connection and orientation.
The first set of windings 210 of the AC power supply 200 is connected to the respective primary legs of the rectifier assembly 14 by three lines 102, 104, and 106. Where line 102 connects at the junction of rectifiers 22 and 24 of the first primary leg, line 104 connects at the junction of rectifiers 30 and 32 of the second primary leg, and line 106 connects at the junction of rectifiers 38 and 40 of the third primary leg. Similarly, the second set of windings 220 of the dual winding AC power supply 200 is correspondingly connected to the respective primary legs of the rectifier assembly 14 by three lines 112, 114, and 116. Where line 112 connects at the junction of rectifiers 52 and 54 of the first primary leg, line 114 connects at the junction of rectifiers 60 and 62 of the second primary leg, and line 116 connects at the junction of rectifiers 68 and 70 of the third primary leg. Thus, the two sets of windings 210 and 220 are effectively connected in parallel between the DC buses 100n and 100p with the series-parallel switches 46 and 48 open.
Since each of these two paths includes passing through the same number of rectifiers and since the voltage of phase 212 has the same instantaneous magnitude and polarity as the voltage of phase 222, the parallel paths share current substantially equally. It is noteworthy the current passes through at least four rectifiers to complete the circuit. Note also, that each of the outboard rectifiers 22, 30, 38, 54, 62, and 70 now have to conduct twice as much current as each of the inboard rectifiers 24, 32, 40, 52, 60, and 68. Therefore, the current rating of these rectifiers is commonly based on the higher duty of the outboard rectifiers, or the rectifier assembly 14 should be physically arranged so that the outboard rectifiers receive preferential cooling.
In addition, the described arrangement of the rectifier assemblies may be supplemented by adding rectifiers or rectifier legs to the rectifier assembly 14. Each supplemental leg similarly connected and conducting current in a similar fashion as described above. For example, a supplemental leg in parallel to each of the legs described above. Such a configuration is depicted in FIG. 1 with first, second and third supplemental legs 122, 132, and 142 respectively. Those skilled in the art will appreciate that such a parallel configuration as described provides effectively double the current rectification capability for the overall system thereby allowing for the use of reduced rating components (e.g., lower current rating rectifiers) or for higher current capability negative DC bus 100n and 100p.
The above described configuration results in the certain rectifiers of each leg being required to conduct more current than others in the leg. Further, in a typical configuration the individual rectifiers may very well be part of a larger package including several individual rectifiers and further may even include interconnections between the individual elements within the package. In consideration of such a typical application, some rectifiers is such a package may be required to conduct more current than others, while others may see varying voltage constraints. This type of configuration forces under-utilization of some components to satisfy the rating requirements of others. It is therefore seen to be desirable to have a rectifier assembly, configured to reduce or eliminate under-utilized components, thereby providing maximal capability within a particular rectifier's ratings.